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How does a battery charger stop charging

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Kal_B

Member
Hello everyone,

I couldn't login for some reason so I had to re-register.

I have an old (very old) pallet lift battery charger that I think is overcharging the batteries and took it apart to see what's going on inside and couldn't find how the charger knows that the batteries are charged. It basically has transformers, circuit board, contactor, a fuse (T-Tron JJN-80) , a breaker and a charge gauge.

In general, what kind of sensing devices are used with theses kind of chargers.

Thanks
Kal
 

Colin

Active Member
To start with . . . many battery chargers are designed for a particular type of battery and must only be connected to that type of battery and only a battery with the voltage as specified.
This type of battery charger works on the principle that it produces a final voltage that exactly matches the highest voltage the battery will reach. This is sometimes called the "floating voltage" or "charged voltage" and will be something like 15.2v for a 12v battery. When the battery reaches this high voltage the current drops to a few milliamp and it sits there "fully charged."
Between the transformer and battery will be a rectifier and maybe a charge controller. If it has a charge controller, this circuit will do all the regulating, but otherwise the number of turns on the transformer is adjusted down the last half-turn to make sure the current drops to almost zero when the battery is charged.
Some batteries with not "gas" when this voltage is reached and this is due to the inclusion of an extra metal (rare element - rare metal) in the construction that increases the "floating voltage" to prevent gassing.
If the charger has a PC board containing electronic components, this may be the cause of the problem. Generally the triac or similar component , is damaged.
 

MaxHeadRoom78

Well-Known Member
Many years ago I built my own, it has to 'see' a battery connected before it starts charging, The current decreases according to the state of the attached battery.
Not sure if I still have the schematic!
Max.
 

spec

Well-Known Member
Most Helpful Member
Hello everyone,

I couldn't login for some reason so I had to re-register.

I have an old (very old) pallet lift battery charger that I think is overcharging the batteries and took it apart to see what's going on inside and couldn't find how the charger knows that the batteries are charged. It basically has transformers, circuit board, contactor, a fuse (T-Tron JJN-80) , a breaker and a charge gauge.

In general, what kind of sensing devices are used with theses kind of chargers.

Thanks
Kal
Hi Kal-B (Kal-A),

All batteries are best charged by the correct voltage and current profile, but lead acid batteries are fairly tolerant compared to other batteries.

In automobiles, for example, the battery is charged with a current limit of whatever the alternator can pump out, normally 40 amps to 120 amps, and at a maximum voltage of around 14.6V.

So, what happens in a car is that the battery gets discharged by the huge current taken by the starter motor, 300 to 1,000 amps, which causes the battery voltage to drop, but once the engine fires up, the alternator fairly quickly replenishes the lost charge.

When the battery voltage recovers to 14.6V the alternator maintains that voltage and the battery takes comparatively little current, perhaps 2A.

When the battery is charged and the engine is running, the battery essentially serves no purpose, because all the car electrical equipment is supplied by the alternator.

So that is how the majority of lead acid batteries are charged, not perfectly.

There is another type of charger, which sounds like the one you mention, that just keeps charging regardless of the battery voltage. If the charge current is fairly low this approach will be sort of OK, but if the current is high it will boil the battery.

With the uncontrolled chargers you are expected to time the charge. I used to work in a garage part time and they had a monster battery charger that was uncontrolled. It put out about 40A and would give your battery a good boiling if you forgot to disconnect it in time.

The solution is to incorporate a voltage limiter, of around 14.6V, on your charger.

The other approach would be to just use a switch mode power supply set to 14.6V, to charge your batteries. Switch mode power supplies are available from eBay and Aliexpress for a reasonable price.



spec
 
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Kal_B

Member
Thanks guys. That gives me some ideas to go back and look at the charger again as I had no clue what to look for.
It really is not my thing but they threw it on my plate and I didn't object (loudly) and though it will another learning experience.

The lift was bought with the charger which is actually installed in a box on the lift with a cord to be plugged in a 110 outlet to charge the batteries. The charger sopped working altogether and when I opened it up I found a melted wire and fixed that. Now it charges but doesn't stop and I heard that they found the batteries boiling in the morning after it was plugged in the night before. So I opened it up again but couldn't find a regulator of some sort but I'll have another look. Basically I will trace the cables from the batteries back to the charger and see where they lead to.

Cheers
Kal
 

spec

Well-Known Member
Most Helpful Member
Just a bit more information about lead acid batteries.

Generally speaking you can trickle charge a lead acid battery at C/20 forever without doing too much damage, so if you had a 40 amp hour battery you could trickle charge it at 40/20= 2A.

spec

PS: this is an excellent site for information on most types of batteries: http://batteryuniversity.com/
 

MikeMl

Well-Known Member
Most Helpful Member
Likely a Lester-type resonant-buck-winding transformer in that charger, similar to the one in my golf cart.
It has an extra high voltage winding that is resonated with an ac capacitor. As the battery voltage rises to nearly the peak of the full-wave rectified high-current secondary winding voltage, the secondary current decreases, increasing the Q of the resonant circuit, which "bucks" the magnetic field in the core, further reducing the secondary voltage, further reducing the current into the battery.

Even though the current "tapers", it will still eventually overcharge a flooded-cell battery if left on 24-7. For that reason, most of these chargers have a built-in clock timer to shut them off after several hours. Newer ones use an electronic timer.

My 36V (18 cell) golf cart charger starts out at a charging current of ~25A, and tapers to ~3A as the batteries reach about 2.4V per cell. The timer setting is not too critical because the batteries can tolerate an overcharge at ~3A for several hours without needing much water.

Previous thread about Lester chargers. Rereading the thread, what I said above I misremembered. I think the description in the old thread is better than what I wrote above...
 
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spec

Well-Known Member
Most Helpful Member
Likely a Lester-type resonant-buck-winding transformer in that charger, similar to the one in my golf cart.
It has an extra high voltage winding that is resonated with an ac capacitor. As the battery voltage rises to nearly the peak of the full-wave rectified high-current secondary winding voltage, the current secondary current decreases, increasing the Q of the resonant circuit, which "bucks" the magnetic field in the core, further reducing the secondary voltage, further reducing the current into the battery.

Even though the current "tapers", it will still eventually overcharge a flooded-cell battery if left on 24-7. For that reason, most of these chargers have a built-in clock timer to shut them off after several hours.
Never heard of that before- once again you live and learn on ETO.:cool:

spec
 

spec

Well-Known Member
Most Helpful Member
Hello again,

I found two of these connected between the batteries and the transformers. How do they work and how test the.
http://www.irf.com/product-info/datasheets/data/50ria.pdf

Thanks
Kal
Hi Kal,

As the data sheet says those devices are thyristors, also called silicon controlled rectifiers (SCRs).

They operate like switches which are turned on by a small trigger current and voltage between their gate and cathode terminals.

SCRs will only trigger when their anode is positive with respect to (WRT) their cathode.

SCRs will only then turn off when the voltage between their anode and cathode is zero volts or is negative. SCRs cannot be turned off by removing the trigger signal from the gate (although there are special gate turn-off [GTO] SCRs which can be turn off at the gate)

There are two modes of operation of SCRs:

(1) Complete half cycle (bang bang)
(2) Phase control- the SCR is triggered on at some point of the input waveform.

In a typical SCR controlled battery charger both modes may be used depending on the design of the battery charger.

In addition, SCRs can act as rectifiers in place on normal silicon rectifier diodes.

Alternatively, normal silicon rectifiers can be used before the SCRs.

In a battery charger, using either of the SCR modes, a circuit monitors the average charge current going into the battery and controls the SCR to maintain a target average current, until a maximum battery voltage is reached when the SCRs will no longer be triggered and the battery will be fully charged.

Note that in both modes the current delivered to the battery is in pulses.

The beauty of this type of charger is the efficiency- little power is wasted in the SCRs.

And in a nutshell, and greatly simplified, that is all there is to SCR controlled battery chargers.:)

spec
 
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Kal_B

Member
Thanks spec. I am starting to search for how to identify the gate kathode and anode but I think I can tell from the wiring. I am inclined to to the following:

The smaller wire going to the circuit board is the gate, the beefier wire is the anode and the bolt/body is the kathode.

Kal
 

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spec

Well-Known Member
Most Helpful Member
Thanks spec. I am starting to search for how to identify the gate kathode and anode but I think I can tell from the wiring. I am inclined to to the following:

The smaller wire going to the circuit board is the gate, the beefier wire is the anode and the bolt/body is the kathode.

Kal
Hi Kal,

As you say, the small wire would certainly be the gate. If you removed one of the SCRs you could do a simple test to confirm which terminal is which.

There are three transformers?

Are there two rectifier diodes in addition to the two SCRs?

spec
 

Kal_B

Member
Hi spec,

There are three transformers.
I didn't see rectifier diodes but they may be covered with heat shrink tubing.
I did a few tests while I was by the charger and was watching youtube but they didn't make sense to me. I'll try again tomorrow.

Thanks
Kal
 

MikeMl

Well-Known Member
Most Helpful Member

Kal_B

Member
Thanks spec.

Any guess as to what those things are on the top right side of the transformer in clear plastic and connected to nuts?

Kal
 

MikeMl

Well-Known Member
Most Helpful Member
Thanks spec.

Any guess as to what those things are on the top right side of the transformer in clear plastic and connected to nuts?

Kal
They might be Klixon-type over-temperature sensors, likely normally-closed.
 

ChrisP58

Well-Known Member
What is the AC input to this charger? When I saw three transformers, my first thought was that it might be a 3-phase input. But then it should have 3 (or 6) SCRs.
 
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